This invention pertains to the art of high temperature lamp assemblies and more particularly to thermal control of the lamp assembly to control the life of the lamp and adapt the lamp to a wider array of end uses.
The invention is particularly applicable to a high brightness light source enclosed in a module or housing and will be described with particular reference thereto. However, it will be appreciated that certain aspects have broader applications and may be advantageously employed in related lamp environments and applications.
High brightness light sources have come under increasing use, particularly in central lighting systems where light is conveyed to a remote location via a light distribution apparatus. The high brightness lighting system is also useful in other environments but one drawback to these types of systems is the associated high temperature. One preferred high brightness light source is an arc discharge source as defined in commonly assigned U.S. Pat. No. 5,239,230 to Mathews, et al. The light source described in that patent provides an effective brightness in excess of 50,000 lumens per centimeter squared and is also intended for extended useful life on the order of 2,000 hours of operation or longer. This particular light source operates at a high temperature, for example, the arc tube operates at 800.degree. to 900.degree. C. Other light sources than the arc discharge arrangement described in the '230 patent are considered high brightness sources. For example, U.S. Pat. No. 5,045,748, issued Sep. 3, 1991 to Ahlgren, et al. is another example of a high brightness light source that operates at an elevated temperature.
Typically, a thin metal foil such as a molybdenum foil is used at a seal interface between an envelope and the electrical supply to electrodes having inner ends disposed in the arc chamber. Exposure of the metal foil to elevated temperatures for prolonged periods of time has been known to result in oxidation and expansion of the metal foil. The seal then opens and vents the envelope so that effective operation of the lamp is terminated. The temperature versus oxidation rate has a threshold value near 350.degree. C. In other words, prolonged exposure of the seal at or above the threshold temperature can result in rapid oxidation. On the other hand, maintaining the seal at a temperature below this threshold value decreases the rate of oxidation and thereby results in longer lamp life.
Continued development of the high brightness light source and its incorporation into a module has required increased emphasis on thermal management. Controlling the overall temperature of the immediate housing also has a desirable impact on lamp life. Preferably, the housing should also address related concerns such as containment of the lamp assembly, alignment and replaceability, as well as ultraviolet and infrared protection.
In the past, attempts to control the elevated operating temperature and its effect on the surrounding environment focused on using fans as a primary source for cooling. Although for some uses this may still be the preferred method for cooling a lamp assembly, blowing air on the high brightness arc tube can adversely effect the light output. The operating parameters of the light source are particularly temperature sensitive so that the light color may be altered by using a cooling fan, and in more extreme situations, start up of the lamp may be effected.